1. Field of the Invention
The present invention relates to a measuring apparatus for measuring a digital signal. More particularly, the present invention relates to a measuring apparatus and a program for measuring timing such as a variation point of a digital signal value and an intersection point of differential digital signals.
2. Description of Related Art
Conventionally, when testing an electronic device, the good or bad of the electronic device is decided by measuring output signals from the electronic device. For example, the good or bad of the electronic device is decided by comparing output signals from the electronic device and predetermined patterns with each other.
Conventionally, such a decision is performed by generating strobe signals in a predetermined period and detecting a value of an output signal in the timing of the strobe signal. For example, the decision is performed by comparing the value of the detected output signal and given H comparison level and L comparison level, converting the output signal into H logic and L logic patterns, and comparing the patterns and expectation patterns.
Moreover, the good or bad of the electronic device can also be decided by evaluating a waveform of an output signal from the electronic device. For example, the good or bad of the electronic device can be decided by whether the timing at an edge of the output signal is within a predetermined range.
The timing at the edge of the output signal can be measured by detecting a signal value near the edge of the output signal, e.g., by means of a multi-strobe (a polyphase strobe) including a plurality of strobes of which phases are slightly different from one another. In other words, the timing at the edge of the output signal is measured by converting the output signal near the edge into H logic and L logic patterns and detecting a phase of a strobe that varies from H logic to L logic, by means of the multi-strobe.
Moreover, a characteristic of the electronic device to be test includes the timing at a variation point of an output signal, the timing at a cross point of differential output signals, and so on, e.g., given from a HIZ (high impedance) level.
The HIZ level and the level of the cross point of the differential output signal are generally smaller than H comparison level and larger than L comparison level. For this reason, it is difficult to detect the timing at the variation point or the timing at the cross point of the differential output signals given from the HIZ level by means of a conventional method. For example, the conventional method can detect the variation point or the cross point of the differential output signal given from the HIZ level by shifting H comparison level or L comparison level by degrees. However, it is difficult to do such a control.